Charles Hudson had a good post this week titled, Marketplaces, Rating Systems, And Leakage.
In it, he talks about leakage in online marketplaces. Leakage defined as a user coming to a marketplace to transact and then completing subsequent transactions off of the marketplace.

Once they’ve acquired a new customer through a service, there’s a significant financial incentive for sellers (Open Table restaurant owners, Uber drivers, Task Rabbit workers) to try to get the user to make their second transaction offline – to avoid paying the marketplace a commission.

But these marketplaces aren’t seeing this type of behavior. They’re seeing that subsequent transactions are staying in their marketplace.

The reason for this is twofold:

The user values the utility of the service (it’s much easier to book a restaurant reservation on Open Table than it is to call, wait on hold, and find they don't have any tables tonight).

As Charles points out, sellers place a high value on reviews from the marketplace. A commenter notes that he once offered to pay for his Task Rabbit project offline and the seller declined. The seller would rather the transaction happen on Task Rabbit so a review gets logged for his work, improving his Task Rabbit reputation. For savvy sellers, a good review on a trusted marketplace is like gold.

Internet marketplaces are always at risk of becoming a lead generation service instead of the central spot where transactions happen. To keep people transacting in the marketplace, it’s important that buyers value the utility of the service and sellers value the reputation gained through post-purchase reviews. Open Table, Uber and Task Rabbit do both of these things well.